Using radioimmunoassay

tissue distribution of melatonin and its seasonal change in the ricefield eel

Monopterus albus Zuiew

was studied. Except for skin and muscle

melatonin was detectable in most tissues of the ricefield eels with different gonadal status. The tissue melatonin levels (pg/mL) declined in the order as follows: eye>whole brain>serum>liver>but>kidney>spleen>heart. In the brain ricefield eel

the level of melatonin was seen higher in the hypothalamus and pituitary

intermediate in the olfactory bulbs

optic tectum and cerebellum

and lower in the telencephalon and medulla oblongata. The melatonin levels in each tissue varied with season during the reproductive cycle